1. Field of the Invention
The present invention relates to a technology for removing mercury contained in an exhaust gas.
2. Description of the Related Art
Exhaust gas generated by combusting coal or heavy oil may contain harmful or toxic material such as soot, sulfur oxide (SOx), nitrogen oxide (NOx), and metal mercury.
Recently, various techniques have been devised for treating metal mercury in combination with denitration equipment that reduce NOx and wet desulfurization equipment that use alkali absorbing solution as an SOx absorbent.
As a method of treating metal mercury in the exhaust gas, a method of using a sorbent is widely known. The sorbent can be an activated carbon or a selenium filter. However, because this method requires sorbent injection devices and larger-scale sorbent collectors, or a special type of absorption removal means, it is costly and not suitable for the treatment of large amount of exhaust gas such as those exhausted from power plants.
To solve these issues, Japanese Patent Application Laid-Open No. H10-230137 discloses a method in which a chlorinating agent is gas-atomized on the upstream side of a denitration equipment at a hot temperature in a flue, mercury is oxidized (chlorinated) on a denitration catalyst to prepare soluble mercury chloride, and the mercury chloride is absorbed in a wet desulfurization equipment installed on the downstream.
A device and a technology for gas atomization in a flue are put into practice by an ammonia (NH3) atomizer in a denitration equipment, and the same scheme can be applicable to gas atomization of a chlorinating agent. However, because hydrochloric gas is highly corrosive, if the chlorinating agent is added in surplus, it causes corrosion of the flue or a downstream device in the system, and therefore, there is a problem that the life of the plant is shortened.
To solve this problem, Japanese Patent Application Laid-Open No. 2001-198434 discloses a system in which a mercury concentration is measured of the exhaust gas obtained after a wet desulfurization is performed, and the amount of the chlorinating agent is adjusted based on the mercury concentration after the desulfurization is performed.
Adjustment of the amount of the chlorinating agent is relatively easy if highly-pure hydrochloric gas is directly used as the chlorinating agent. However, it is costly and not economical for the treatment of large amount of exhaust gas.
Furthermore, regarding spraying a chlorinating agent, there have been few materials that can be directly sprayed at an operating temperature of the denitration equipment (in a range between 350° C. and 420° C.). If neutral salt slurry such as sodium chloride (NaCl) is used, because it not decompose at the operating temperature of the denitration equipment, a desired effect cannot be obtained, possibly causing clogging.